ath9k: simplify beacon configuration for beaconing vifs

As of now beacon configuration is being called multiple times
in bss info change notification. This patch avoids multiple
configuration and make it simpler.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>